Adding interceptors resulted in stripped security headers.

[improbable-ludwik]

Hello

When we added the grpc_retry.StreamClientInterceptor() and grpc_retry.UnaryClientInterceptor() grpc.DialOption to a grpc.DialContext we saw our service throw error like:

{code = Unauthenticated desc = request missing 'Authorize' credentials}

suggesting that header information was being stripped by the middleware.

[johanbrandhorst]

This should be easy enough to test. If you could contribute a test that confirms this bug it should be easy to fix!